Most commercial water contains alkaline earth metal cations such as calcium; magnesium, and various anions such as the bicarbonate, carbonate, sulfate, silicate, phosphate, etc. Combinations of these ions in concentrations exceeding their solubility form precipitates which deposit in the system, e.g. boiling water system. As these ions precipitate from the water, they can form scale on the surfaces which prevents effective heat transfer, and increases the corrosion process. The accumulation of scale, particularly in boiling water systems, is an expensive and tremendous problem in most industrial water systems, causing delays and shutdowns, requiring many manhours in cleaning and removal.
The term "scale" as used herein, includes deposits formed on the surface in contact with aqueous mediums due to the precipitation of metal ions from the water. Water containing ionized scale forming components is used in many domestic and industrial applications for a variety of purposes and because the water is from a variety of sources the scale forming components that precipitate on the surfaces also vary in quantity and type. It has been found, however, that the deposition of scale on the surfaces in contact with the water can be substantially inhibited by utilizing a certain composition to effectively inhibit the formation of scale. The inhibitor present in the water reacts with the scale forming components and alters the crystalline character and amount of precipitate formed, thus inhibiting scale formation.
The use of various scale forming materials to prevent precipitation by inactivating the cations with certain compounds, e.g. phosphonates, is known in the art, see for example, Hatch and Rice, "Industrial Engineering Chemistry", Vol. 31, page 51 and "Journal of Physical Chemistry", Vol. 44, No. 5, (May, 1940). More particularly, various organic phosphonates, including the nitrogen-containing phosphonic acids, especially the N-substituted phosphonic acids have been used in aqueous sytems not only as corrosion inhibitors, but as scale preventing agents. These materials, however, are more difficult to obtain and have a tendency to precipitate in acidic mediums. Other carboxy-containing acids are the phosphonosuccinic acids which have been used as boiler-water scale inhibitors; see German Specifications 2,225,645 and 2,015, 068. Moreover, combinations of various phosphonates with organic polymers have been used in boiler water as taught by U.S. Pat. No. 3,959,167. Other phosphonates in combination with certain amines are disclosed in U.S. Pat. Nos. 3,666,664 and 4,011,171. Thus, the current technology for preventing the deposition of scale by using organophosphonic acids is well documented and it has been demonstrated that phosphonates in boiler water will reduce the amount of scale on the heat transfer surfaces. The phosphonate treatment promotes the formation of a fluid sludge in the water which can be effectively conditioned and removed by boiler blowdown.
It is, therefore, an object of this invention to provide a novel composition which can be used to inhibit scale and promote the formation of fluid sludge in aqueous systems and particularly in boiler water systems. It is another object of this invention to provide a unique combination of a substantially water soluble polymer such as an acrylic acid polymer and a particular phosphonotricarboxylic acid. These and other objects of the invention will become apparent from a further and more detailed description of the invention as follows.